1. Field of the Invention
This invention relates to cosmetic compositions, and, more particularly, to a cosmetic composition for rejuvenating the appearance of skin with substantially no skin irritation.
2. Description of the Prior Art
Cosmetic compositions having an alpha- or beta-hydroxy acid (AHA/BHA) as the active ingredient are well-known in the art. These compositions are useful for treatment of the skin, particularly for anti-aging, improvement in skin tone, reduction of fine line, enhancement of moisture, and development of a smooth skin. Application of such compositions generally results in a younger-looking skin as new cells replace the old. Smith, in Soap/Cosmetics/Chemical Specialties, Sept. 1993, page 54, addressed the question of whether an AHA must be in the acid form to be effective in promoting exfoliation and skin renewal. Various AHA and BHA compounds were examined for their ability to increase cell renewal at different pHs, with similar results observed for all acids tested. As the pH increased, the ability to stimulate cell renewal diminished; in fact, at a pH above 6, very little stimulation of cell turn over was observed with any AHA material. For the acids tested, a maximal stimulation of renewal was observed at a pH of about 3, as shown in Table 1 below.
TABLE 1 Relationship between Cell Renewal, Irritation & pH for Various Acids Tested TEST MATERIAL pH CELL RENEWAL* IRRITATION* 4% Lactic acid 3 35 2.8 5 24 2.1 7 13 1.2 4% Glycolic acid 3 34 2.9 5 23 2.1 7 10 1.1 4% Salicylic acid 3 42 3.0 5 28 2.3 7 12 1.2 5% Citric acid 3 18 2.3 5 14 2.1 7 8 1.1 *A higher number indicates improved cell renewal/more irritation
Accordingly, to achieve improvement in skin condition, it has been found necessary for the user to tolerate the skin irritation caused by the acid present in the product. Thus, skin irritation is a major concern to formulators of alpha and beta-hydroxy acid-containing products, particularly at acid loading levels which can deliver faster and more effective skin peeling. More irritation, however, is perceived by the user as being a more effective treatment.
Scott and Yu, in EP 671162, have described an amphoteric composition containing AHAs and their esters and salts for topical application for the treatment of cosmetic conditions and dermatological disorders. The inclusion of an amphoteric compound in the composition provided a stable composition having a pH closer to that of skin.
Usumura, in U.S. Pat. No. 5,512,277, described a method of removing keratotic plugs by applying a keratotic plug remover composition onto the skin, and peeling off the composition after the composition was dried, and, particularly, wherein the active ingredient of the composition was a cationic polymer compound such as polymethacryloylethyl trimethylammonium chloride.
Chaudhuri, in U.S. Pat. No. 5,736,128 also described a cosmetic composition for rejuvenating the appearance of skin with reduced or minimal skin irritation which included an anionic copolymer of methyl vinyl ether-maleic acid as the active ingredient in an aqueous, alcoholic or aqueous-alcoholic solution of 0.5 to 25 wt. % of the copolymer at a pH of 1.5 to 5.
U.S. Pat. No. 5,037,924 described the preparation of terpolymers of maleic anhydride, a C.sub.1 -C.sub.4 alkyl vinyl ether and isobutylene, for use as denture adhesives in the form of their mixed salts. These terpolymers were made by precipitation polymerization in the presence of a cosolvent system of ethyl acetate and cyclohexane. The resultant terpolymer had a low molecular weight of about 30,000 to 400,000, and contained residual ethyl acetate and cyclohexane in the terpolymer.
Accordingly, it is an object of this invention to provide a new and improved cosmetic composition for the rejuvenating the appearance of skin which produces excellent skin compatability with substantially no skin irritation, and with the environmental benefit of a residual solvent-free system.